In clean-environment manufacturing industries, such as the manufacturing of flat panel display devices and semiconductors, there is a need to handle large amounts of in-process inventories. In these industries, discrete vehicle-type transports, such as monorail vehicles or automated guided vehicles, have prevailed, especially in the front-end of clean manufacturing processes. Manually operated vehicle-type carts have served such industries as well.
Historically, a conveyor methodology of transport has not been accepted in these industries, where ultra-clean handling of material is required. Recent advances in conveyor technology have made conveyor systems more attractive for use in clean-environment manufacturing industries. Some of these advances include: improved cassettes for riding directly on the conveyor and limiting the bounce in wafers carried thereon; standardized runners for accepting different-sized carriers to reduce the need for customized conveyor belt sizes; providing distributed control to convey items substantially independently without collisions; selective control of carrier movement to reduce gaps between carriers or items being conveyed; and reducing particulate generation through the use of clutches in drive wheels.
Typically, multiple wafers or substrates, several times the amount in-process, are stored for access during the manufacturing process, regardless of the transport mechanism. Stockers have been employed with conventional discrete-vehicle transport systems. Such systems employ a central robot which has access to plural floor-mounted storage cabinets providing bins, shelves or other storage space. A work-piece to be stored is transported to a designated transfer position on the conveying system. It is then retrieved by the stocker robot, which is then manipulated to place the work-piece in the appropriate storage location. As is evident, no parallel work-piece manipulations can be executed by the stocker robot while this storing activity is underway. The same limitation to single work-piece manipulation exists when the stocker robot is in the process of retrieving a work-piece. The stocker becomes a bottle-neck for work-piece flow.
Another deficiency associated with stockers pertains to the amount of floor space which such systems typically occupy. Floor space adjacent a conveying system and in a manufacturing environment may be at a premium. If multiple stockers are provided as a means for lessening the bottleneck in work-piece flow as compared to the use of a single stocker, that much more floor space is consumed. The cost of the overall system is significantly raised as a consequence.
The introduction of the clean conveyor system and the vehicle transport method by the Applicants requires a storage and buffer solution for work-in-process material that is in harmony with the conveyor transport methodology, and is the equivalent of the above-mentioned stockers in terms of storage capacity and accessibility to the conveying system.